Is email toolbar customization gone in iPadOS 26.2?

Customisation of the iPadOS 26 Mail toolbar is still available - although the method of accessing the customisation box is considerably more convoluted than earlier versions of iPadOS...

1) Ensure that you are using the iPadOS 26 Windowed Apps mode:

Settings > Multitasking & Gestures - select Windowed Apps

2) Open the Mail App; maximise the Mail App window (use the traffic-light multitasking button is necessary; tap the green button to maximise the Mail App window).

3) Swipe-downwards from the top-centre of the screen to expose the menu bar:

4) From the menu bar, tap View to expand the associated menu - then select Customise Toolbar:

5) Edit the Mail toolbar as required:

Addendum to my earlier reply.

Step #2 of my earlier reply, maximising the Mail Window, is not absolutely necessary. You can still expose the menu-bar for current foreground Windowed App using the downward-swipe from top-centre of the screen - as illustrated here:

It is, however, necessary to have selected the Windowed Apps mode from Multitasking settings. The menu-bar is not accessible if running the iPad in Full-Screen Apps mode:

Additional information can be found within the iPad User Guide - this being a comprehensive and often overlooked source of information for iPad and iPadOS. Here is a direct link to the relevant section of the Guide:

Use the menu bar on iPad - Apple Support